Solid foods which normally contain moisture in their interiors, such as fruits, vegetables, cereals and grains can be preserved by drying. this drying is conventionally done by purely convective means, in which the solid is exposed to a moving stream of heated air which evaporates the water contained by the solid and carries it away. In conventional belt dryers, the solid product is moved on an air pervious steel belt that passes through a dryer where warm air is blown upwardly through the belt. In fluidized bed dryers, solid particles are suspended in an upward flow of heated air, which dries the particles and carries the moisture away.
Because most food products are sensitive to high temperatures, food dryers operate at low temperatures, often below 100.degree. centigrade. At such low temperatures, drying rates are slow. To achieve a moderate rate of dried product output at low temperatures, a belt dryer must have a long length, and a fluidized bed dryer must have a large volume. The disadvantages of such equipment include high cost, extensive space requirements, and great heat loss. This last problem is significant in the food processing industry, which is a major energy user.
Convective drying removes moisture from the surface of the solid through evaporation. While this occurs, moisture migrates from the inside of the solid to its surface where it is removed by the passing heated air. But, as total moisture content is reduced, the rate of moisture migration declines. This in turn causes a decline in the drying rate, since less moisture is moving to the solid's surface where it can be evaporated off. The surface temperature of the solid then increases because of the decline in evaporation rate. This temperature increase is undesirable because of the potential damage to the temperature-sensitive solid. Such damage includes cracking, splitting, uneven cooking, and other undesirable effects on the quality of the product.
The application of microwave energy during thermal convection drying has been found to accelerate the drying rate even at low temperatures. Because water is an effective absorber of microwave radiation, food solids tend to overheat if microwaves are applied while too large a quantity of moisture remains therein. For this reason, microwave radiation is best applied only after the moisture content of the solid has already been substantially reduced. At this point, where moisture migration rates are low, the use of microwave energy is advantageous in that it will heat the internal moisture and drive it out of the solid.
Up to now, however, the use of microwaves in food dryers has had a major drawback--the requirement that substantial amounts of externally generated electricity be purchased. Since food drying incurs great energy demands, this added energy expense is critical. Furthermore, while microwave drying has been used, it has not been combined with convective drying in the most effective manner, with consideration being given to the effects of both processes on drying rates and surface temperatures.